Arceus
Arceus is the creator and god of all pokemon. In this story, he is unaffected by the curse and retains his original nature. As the creator of all pokemon, he feels disgusted and tormn over the devastation that befell his world. As a result, he is roaming the world, assisting the human and pokemon survivors in clearing out large areas of the cursed pokemon. Although he sees all pokemon as his children, even the cursed ones, he prefers to grant them the peace of death than see them in this torture. Arceus plays a vital part in the pokemon world, and is believed to be the key to either saving or dooming the pokemon world completely. Ash's ultimate plan was to corrupt Arceus, which would ensure that the rest of the world would follow. However, Arceus proved too powerful to succumb tot he curse or be defeated in battle, therefore Ash sought out to weaken it first, after finding out that it has a fundamental connection to the world, and the more the world succumbed to the curse, the weaker Arceus was becoming. In particular, the more legendary pokemon that succumbed to the curse, the more Arceus was getting weaker. At some point, enough pokemon were cursed and Ash decided to face Arceus. He enlisted all corrupted legendary pokemon, Nihilego, Guzzlord, Poipole, Naganadel,Blacephalon, Lunana in his party, and lastly Giratina and Necrozma on their own will. Since Necrozma was not corrupted from the beginning of the curse, but did so after, when it transformed to Dawn Wings Necrozma form, Ash got the idea that Arceus would be corrupted too if it would be given a plate of one of the corrupted types. During the confromntation, Arceus proved very strong for Ash's six legendary pokemon, however he managed to push Arceus in the Distortion world, where Necrozma and Giratina kept him imprisoned. After Arceus' imprisonment, a lengthy battle took place between Arceus and Necrozma and Giratina on the other hand. Seeing that he couldn't do much, Ash left the distortion world and aimed to make more legendary pokemon succumb to the curse, so that Arceus would become even weaker and eventually lose to the cursed duo. After the events of Ash's betrayal in Saffron, Arceus grew weaker. Ash went back into the distortion world, but Vergil, Cole, Brock, among others also found out about this and opened a gate to the distortion world as well. THere they fought Ash, who was unsuccessful in turning Arceus evil. However, while his freedom would give hope for restoring the pokemon world, Arceus perished there along with other cursed legendary pokemon. Before he died though, he entrusted Vergil (Or Cole) with the jewel of life, a very powerful artifact whic will be important in the future. After this, there was only one way for Ash to corrupt the pokemon world. He had to do it region by region, with the most regions already having fallen, and in Kanto the only way to do that was to have a pokemon championship and win it.